The standard technique of Marketo de-duplication for new-records is by email address. If records already exist in the marketo with similar email address then record will be updated with new/missing information only and if email address is not found then new record will be created.

Do you know you can customize the de-dup rule in Marketo?
See, de-dup customization is only viable when you have some other unique field (like email address) tagged with your lead then only it will be useful for your business.

For example: I am into education domain and one lead can be tagged with the college name only once then you can use college name as your secondary key for de-duplication rule

If you set up a custom de-dup rule, Marketo will match on email as primary key, and another field as a secondary key. For example, if you add college name as a secondary de-dup field then see the scenario below:

Case 1: When lead entered the database for the first time, email address was “xyz@abc.com” and college name was “XYZ”

Marketo Action - New lead is created

Case 2: Lead entered the database with same email address “xyz@abc.com” but college name is different. College name is “ABC” then de-dup rule will check primary key first that is email address {Primary key matched} then it will check secondary key, college name which is different {Secondary key mis-matched}
Marketo Action: New lead is created with same email address but different college name

Case 3: Lead entered the database with same email address and college name as case 1 but this time it’s also tagged with mobile number and home address. Marketo will match primary key and secondary key {Match perfect}
Marketo Action: Lead created with case 1 will be updated with new information

Custom de-dup rules apply to list uploads, form submissions and most API integration but It does not apply to the native CRM sync like salesforce / Microsoft dynamics. These CRM uses their own de-dup rule and that is based on CRM ID.

I hope this blog will help you to understand Marketo de-duplication rule.

Thanks,

Sant Rathore